The present invention is directed to a method and apparatus for high-throughput, nondestructive measurement of hydrolytic stability of polymers. The invention is particularly advantageous as a method and apparatus for fluorescent measurement of degradation of polymer molecular weight.
One important consideration in the area of polymer chemistry is long-term stability. For example, thermal variation, sunlight and hydration can cause crazes and cracks in polymers with a resulting decrease in structural integrity. Generally, two types of aging occur: physical aging and chemical aging. Physical aging often occurs as a result of the formation of water pockets in a polymer, and is associated with crack formation, voids and loss of ductility leading to premature mechanical failure (Golovoy, A. and M. Zimbo, Polym. Eng. Sci., 29:1733-37 (1989)). Chemical aging occurs as a result of hydrolysis of the polymer chain, and is associated with a reduction in polymer molecular weight (Pryde, C. A. and M. Y. Hellman, J. Appl. Polym. Sci., 25:2573-87 (1980); Golovoy, A. and M. Zimbo, (1989); and Knauss et al., Polym. Mater. Sci. Eng., 72:232-233 (1995)).
Studies of hydrolytic degradation of polycarbonates indicate that while initial rates appear to be governed by first order reaction kinetics, over time there may be an autocatalytic effect due to an increase in phenolic end groups as the polymer degrades (Golovoy, A. and M. Zimbo (1989); Bair, H. E., et al., J. Appl. Polym. Sci., 26:1777-86 (1981)). Thus, an important factor with respect to hydrolytic stability of solid polycarbonates is the degree of end-group capping. Capping of phenolic end groups by chain terminating agents minimizes the polar group content of the polymer and thereby increases hydrolytic stability (Pryde, C. A. and M. Y. Hellman, (1980)).
Also, additives may increase the rate of hydrolysis. It has been shown that chain hydrolysis is increased by base, acid, polar compounds and even by neutral species present in polymers (Golovoy, A. and M. Zimbo (1989); Pryde et al., Polym. Eng. Sci., 22:370-75 (1982)). For example, brominated polycarbonate flame retardant accelerates hydrolysis (Golovoy, A. and M. Zimbo (1989)). In contrast, the hydrolytic stability of copolyester-carbonates is improved by the addition of additives (U.S. Pat. No. 4,436,879), and the hydrolytic stability of polyurethanes is improved upon addition of carbodiimide (Schollenberger, C. S. and Stewart, F. D., Angew. Makromol. Chem., (1973) 29-30, 413-30). Hydrolytic stability of polymers may also be improved by the character of the polymer blend. Incorporation of phosphorous compounds into a polymer backbone may increase or decrease hydrolytic stability. For example, incorporation of phosphorous as random copolymers of polycarbonate and bis(4-hydroxyphenyl)phenylphosphine oxide (Knauss, D. M., et al., Polym. Mater. Sci. Eng., 71:229-30 (1994)), or as polycarbonate formed by the reaction of aromatic trihydroxyphosphorous compounds (U.S. Pat. No. 4,680,370), or by condensation with bishydroxyalkylphosphine oxide (U.S. Pat. No. 4,556,698) is associated with increased hydrolytic stability. In contrast, phosphorous-modified polyestercarbonate resins display decreased hydrolytic stability (U.S. Pat. No. 4,474,937).
There are few methods available to evaluate the stability of polymers to chain hydrolysis. Outdoor weathering data is useful, but requires multi-year exposure times. Autoclaving samples provides for accelerated weathering. Still, because of physical changes that occur at temperatures much above 100xc2x0 C., autoclave tests cannot by themselves be used to predict long-term hydrolytic stability.
Conventional techniques to determine the extent of hydrolytic degradation of a polymer involve dissolution of the sample and molecular weight determination by gel permeation chromatography (GPC). There are, however, significant drawbacks associated with this approach. The method is destructive, in that it requires the sample be dissolved, and therefore is inappropriate for field (in situ) studies of polymer integrity. Also, the method is time-consuming to perform and generates non-biodegradable waste.
Other methods to monitor polycarbonate integrity include characterization of sample xe2x80x9chazingxe2x80x9d and ductility. Hazing refers to the tendency of worn polycarbonate samples to become refractive to visual light. Hazing is typically evaluated by measuring light transmittance through a sample (see e.g. U.S. Pat. No. 4,436,879). The use of hazing for analysis of polymer hydrolysis is limited in that it is difficult to establish quantitative correlation between the amount of hazing and a loss of polymer chain length or molecular weight in the presence of variations in sample thickness (flatness). Variations in sample thickness increase the error in transmission measurements of haze or other optical properties (see e.g. Ingle, J. D, and S. R. Crouch, Spectrochemical Analysis, Prentice Hall, Englewood Cliffs, N.J. (1988); thus, for haze measurements sample thickness should be controlled (U.S. Pat. No. 4,436,879). Also, samples must be thin enough to enable efficient and measurable transmission of light through the sample. Generally, polymers characterized by a high light transmittance exhibit improved clarity and high levels of hydrolytic stability. Conversely, polymers with poor light transmittance exhibit poor clarity and poor hydrolytic stability (U.S. Pat. No. 4,436,879). Thus, samples with decreasing yellow color, as measured by hazing, tend to exhibit increased hydrolytic stability (see e.g. JP08188707; JP08245782; and JP08208829).
Ductility refers to the relative pliability of a polymer sample. Loss of ductility is typically measured by a pellet gun impact test. Because of its destructive nature, the pellet gun test is of little use for in situ evaluation (field tests) of polymer aging. In addition, a loss of ductility as measured by the pellet gun impact test is not necessarily correlated to hydrolytic degradation or a decrease in polymer molecular weight.
Poor hydrolytic stability results in other limitations on the mechanical properties of polymers exposed to a hydrolytic environment for extended time. For example, poor hydrolytic stability may lead to a decrease in reduced viscosity of the polymer upon exposure to a hydrolytic environment (U.S. Pat. No. 4,595,404). Tests applied to evaluate mechanical properties related to hydrolytic stability include tensile modulus, tensile strength, elongation, notched Izod impact strength, viscosity and others (see e.g. U.S. Pat. Nos. 4,348,500 and 4,595,404). Still, many of these mechanical tests are destructive.
Thus, there is a need for a test which enables the nondestructive measurement of the hydrolytic stability of polymers. The test should be sensitive and capable of measurements of samples of different geometries and thickness, such that samples are not limited only to flat sections commonly used for transmission measurements. Also, the test should provide a reliable assessment of polymer molecular weight.
Preferably, such a test would enable noninvasive, high-throughput testing of samples in the field, as well as the rapid analysis of multiple laboratory samples, as for example, in a combinatorial library.
The present invention is directed to a method and apparatus for rapid, high-throughput, nondestructive determination of hydrolytic stability of polymers. In one aspect, the invention comprises a method for the nondestructive determination of hydrolytic stability of polymers comprising irradiating a polymer sample with light of at least one substantially monochromatic excitation wavelength, wherein the excitation wavelengths used for irradiation are selected based upon an analysis of multiple emission spectra of the polymer irradiated at multiple excitation wavelengths; collecting fluorescent radiation emitted by the irradiated polymer sample for each excitation wavelength; monitoring emitted radiation which is substantially dependent upon polymer molecular weight; comparing the monitored radiation emitted by the polymer sample with the monitored radiation emitted from a similarly irradiated control polymer known to be substantially free of hydrolytic cleavage; and correlating a decrease of the emitted radiation substantially dependent upon molecular weight to hydrolysis of the polymer sample.
Another aspect of the invention comprises an apparatus for the nondestructive monitoring of polymer hydrolysis comprising a light source which irradiates a polymer sample with at least one wavelength of substantially monochromatic light; a probe which transmits light from the light source to irradiate the polymer sample and collects fluorescent light emitted from the sample; and a detector, wherein the detector monitors radiation emitted by the sample comprising fluorescence substantially dependent upon polymer molecular weight. Also included in the present invention are systems for performing the methods of the invention.
The methods and apparatus of the invention are noninvasive and, thus, suitable for tests of hydrolytic degradation of polymer samples which are components of a larger structure, or for combinatorial libraries in which multiple samples are arranged in an array format.